The cold cathode fluorescent lamp (CCFL) was developed in the late twentieth century and is currently used in many products such as the backlight source for flat panel display. The CCFL, different from a filament lamp, is a discharge lamp composed of a low-pressure mercury emitting 253.7 mm ultraviolet light. The ultraviolet light is emitted from mercury molecules impacted by the discharged electrons, thereby generating more electrons bombarding the fluorescent materials coated inside the tube. The CCFL is characterized by its longer lifetime and lower operating temperature than that of a filament lamp. Thus, less energy is consumed and the danger of burning down is reduced. Moreover, the CCFL emits uniform and stable luminance density of light. The energy of the emitting ultraviolet light is generated by electrons falling back to their ground state due to energy gaps.
Flat panels such as liquid crystal display (LCD) panels are popular worldwide and increase the demand of CCFL because flat panel displays usually are not able to illuminate light by their own. In addition to LCD panels, scanners, fax machines, and indicators all utilize CCFL tubes. The CCFL tube is typically small, light, cost effective, have a long lifetime, and in particular, consumes little power which is important to mobile apparatuses, i.e. digital cameras and mobile phones. With the advent of technology, dimming of CCFL can easily be controlled. Additionally, the circuits used to stabilize the lighting up and turning off of CCFL can be easily integrated into a system.
Circuit design of a CCFL controller should be based on the characteristics of the CCFL tube, which are very different from those of filament bulbs. There are usually two steps need to be executed in order for CCFL tubes to emit light. First, the electrical system ignites the CCFL tube, i.e. to excite or to ionize the electrons distributed in the mercury gas. This requires very high amplitude voltage, which is usually several times the amplitude of the voltage applied in an ionization step. Next, the electrical system needs to maintain a stable alternating current to support continuous illumination. Since the CCFL tube is operated by an alternating current, the voltage passes the zero point twice in every cycle of the alternating current, including an ignition step that is necessary for every cycle. The power source for CCFL usually has a voltage around 300˜400 Vrms with sinusoidal waveform, a current around 5˜6 mArms, and the frequency in the range from 25 KHz to 100 KHz. The power source requires a peak over 1000 V to activate the ionization of the CCFL. A major difficulty in designing the CCFL backlight inverters is the incorporation of the very different characteristics of the ionization step and the maintenance step.
During ionization step, the ignition voltage increases to a level high enough to induce the avalanche reaction which is several times the typical forward operating voltage. The output voltage for illumination is roughly proportional to the average current. The CCFL tube exhibits a positive resistance and usually causes ambient temperature to increase. Meanwhile, the current control issue requires attention. After ionization, the CCFL tube exhibits a negative resistance if supplying a current that exceeds 1 mA. A current source is usually applied to drive a load with characteristics similar to CCFL tubes because the illumination of CCFL is primarily controlled by the average value of the applied current. The ignition voltage rises to the avalanche level until ionization is reached. Then, the voltage collapses to the operating voltage for immediate illumination.
Normally, CCFL drivers, also known as inverters or converters, utilize an electromagnetic transformer in self-resonant mode. A variety of structures are available for CCFL drivers, such as current-fed push-pull resonant inverters, current-fed Royer oscillators, half-bridge converters, and full-bridge converters.
The push-pull converter in FIG. 5A includes two transistors Q1 and Q2 alternately switches on for time periods Ton, causing the transformer core to provide an alternating voltage polarity to maximize its efficiency. The transfer function follows the basic pulse width modulation (PWM) formula and a factor of 2 is added because the two transistors alternately conduct for a portion of the switching cycle. A dead time is inserted in order to avoid a short circuit which can be the result when two transistors conduct at the same time. In a push-pull converter design, because the frequency of the ripple is twice the operating frequency, the size of the LC filters is reduced. However, the main disadvantage of the push-pull converter is that a center-tap connection transformer is required.
The half-bridge converter in FIG. 5B includes two transistors, Q1 and Q2, two capacitors, C1 and C2, and two ultra-fast diodes, D1 and D2. The diode D1 connects to the transistor Q1 in parallel, and the diode D2 connects to the transistor Q2 in parallel. One terminal of the capacitor C1 connects to one terminal of the primary winding of the transformer, and the other terminal of C1 connects to the positive power supply. One terminal of the capacitor C2 connects to the terminal of the primary winding of the transformer which also connects to capacitor C1, and the other terminal of C2 connects to the negative power supply. The input voltage is equally divided by the capacitors C1 and C2 so when either one of the transistors turns on, the transformer primarily sees Vin/2. Consequently, there is no factor 2 in the transfer function of half-bridge converter design. In a full-bridge converter design, four transistors are utilized without capacitors. Therefore, all voltages are shared equally between the transistors so that the maximum voltage can approach to VIN.
In U.S. Pat. No. 4,607,323 to Sokal et al. titled “Class E High-Frequency High-Efficiency DC/DC Power Converter”, a Class E switching-mode dc/dc power converter, sometimes also known as a Class E switching-mode dc/dc power inverter, is disclosed. The entire disclosure is incorporated herein for reference. This converter operates at high frequencies, and has low power dissipation and low second-breakdown stress during turn-ons and turn-offs. In U.S. Pat. No. 5,818,709 and U.S. Pat. No. 5,834,907 to Takehara titled “Inverter Apparatus” and “Cold Cathode Tube Operating Apparatus with Piezoelectric Transformer”, an inverter apparatus comprises a serial resonance circuit, a voltage feedback, and a CCFL apparatus with piezoelectric transformer, are disclosed respectively.
The stability of the current driving, and the extra components required in the prior art, i.e. the multiple transistors or the expensive piezoelectric transformer, all play important roles in the construction of a CCFL driving apparatus. It is an object of the present invention, in view of improving the efficiency and the cost effectiveness of the driver for the CCFL, to provide a driving apparatus and circuit for LCD backlight with minimum number of components and with stable supply of current such that the overall cost of LCD display apparatus can be further reduced.